Sleep is regulated by the activity of histamine cells

''Sleeping Dogs, And Why'' Brain activity in narcoleptic dogs has helped solve some of the mysteries of sleep and waking. Brain
chemicals thought to have the same role appear instead to preside over different aspects of sleep. AWAKE -- Brain cells that produce
histamine and norepinephrine are active during waking. PARTLY ASLEEP -- In cataplexy, a semisleep state in which muscle tone is lost
but arousal maintained, histamine levels are normal; norepinephrine is reduced. SLEEPING -- Both histamine and norepinephrine are
silent. Histamine regulates arousal, while norepinephrine appears to control muscle tone.
http://www.nytimes.com/2004/06/15/health/teasing-apart-chemicals-scientists-unlock-one-more-of-sleep-s-secrets.html